Acoustic behavior relies on information conveyed by projection systems from the inferior colliculus to the medal geniculate body and then to the auditory cortex. The long term goal of this research is to evaluate auditory information processing in these projection systems. The proposed research will examine mechanisms by which neurons analyze spectrally and temporally complex sounds. Special attention is placed on anatomical connections and physiological properties which give rise to combination - sensitive neurons. These neurons, which combine inputs responding to different frequencies in the appropriate temporal relationships, perform computational analyses of complex sounds. The proposed studies examine these in the mustached bat auditory system, where they are best understood. The initial experiments describe, using physiological recording, functional properties and organization of combination-sensitive neurons in the inferior colliculus. These experiments will guide retrograde tracing studies that identify auditory brain stem nuclei which may contribute to the combination-sensitive response of collicular neurons. Physiological recordings will be obtained from auditory brain stem regions identified in the preceeding anatomical experiments. These will provide examine whether combination sensitivity originates in inferior colliculus neurons, or at lower levels. An anterograde tracer will be placed in an auditory brain stem region that appears to contribute to combination sensitivity in the inferior colliculus; this can confirm a potential contribution to inferior collicular combination sensitivity. Finally, chemical inactivation experiments will test the presumed role of brain stem nuclei in creating these response features in the colliculus. Because the thalamic reticular nucleus appears to contribute to combination-sensitive responses in the medial geniculate body, its functional properties and anatomical connections will also be examined. Acoustically-guided behavior, including speech and other forms of acoustic communication and sound localization, often relies on neural mechanisms which combine information across different frequency bands. Combination-sensitive neurons in the mustached bat form a major pathway from the inferior colliculus to the medial geniculate body, and then to the auditory cortex, in which temporal information is encoded. In humans, structural irregularities in the medial geniculate body have been implicated in language disorders characterized by temporal processing deficits. Studies of mechanisms creating combination-sensitive neurons in mustached bats may provide a deeper understanding of how analagous speech processing systems operate in humans.